Question

A girl holds an orange at rest in her hands. One force in an action/reaction pair is the force of earth’s gravity pulling down on the orange. What is the other force in the action/reaction pair of forces?
A. The force of hand pushing up on the orange
B .The force of earth pushing up on girl’s feet
C. The force of orange pushing down on the girl’s hand
D. The force of orange pulling up on the earth

Answer 1

Use the concept of Newton’s third law of motion. The object is in contact with the surface of the girl's hand. Recall the force between an object and a surface in contact with each other. Also check what will be the direction of that force.

Complete step by step answer:
It is given that a girl is holding an orange at rest in her hands.
The force form the action/reaction pair acting on the orange is earth’s gravitational force which pulls the orange in the vertically downward direction.
According to Newton’s third law of motion, there is always an equal and opposite reaction force for every action force. These two action and reaction forces always act in pairs and cancel each other as they are in the opposite directions.
Since the earth’s gravitational force is pulling the orange in the girl's hand in downward direction. There must be some equal and opposite balancing force in the upward direction to balance the earth’s gravitational force on the orange.
The surfaces of the hand and the orange are in contact with each other.
When an object in contact with a surface, the surface exerted a normal force on the object in the upward direction perpendicular to the surface.
Hence, the surface of the girl's hand will exert a normal force on the orange in the upward direction to balance the earth’s gravitational force on the object.

Therefore, the reaction force is the force of hands on the orange trying to push the orange in the upward direction.

Hence, the correct option is A.

Note:
The students may think that there must be normal force by the surface of the earth on the feet of the girl in upward direction to balance the gravitational pulling of the earth on the orange. But in order to balance the force on the orange, the reaction force also must be on orange and not on the girl's feet. The earth exerts force on a girl's feet but it is not the reaction force on oranges.